The invention relates to a pitched cable sheathed with a plastic film over a defined length.
The term "pitched cable" here and below designates a wire or a stranded assembly of wires around which is wound a single wire at a constant pitch, where the single wire is called the pitch helix. When seen in longitudinal and axial section, such cables assume the shape of a gear rack and therefore mesh with pinions of the same pitch and upon their longitudinal displacement rotate a pinion or vice-versa are displaced themselves upon pinion rotation.
Compared to rigid gear racks, pitched cables offer the advantage of flexibility while nevertheless are capable of transmitting mechanical tensional and compressive forces. Therefore, the pitched cables are laid in arcs once minimum radii have been observed. However, being flexible, the exploitation of their gear-rack effect requires guiding them in a tube which is cut open at the site of the pinion engagement.
Typically, however, only a limited section of a pitched cable is used in its gear-rack function and the remainder of the cable merely serves for force transmission in the longitudinal cable direction which, as already stated, is laid in arcuate manner. Such an application, for instance, is in the drive mechanism of automobile sunroofs, as disclosed in U.S. Pat. Nos. 2,985,483; 3,572,822; 3,863,979; 3,976,325; 4,081,926; and 4,469,371, the disclosures of which are incorporated herein by reference.
As a rule two lengths of pitched cables are used in a sunroof, which engage a pinion on its opposite sides on the front cross-side of the roof opening. When the pinion rotates, for instance by means of a handcrank, the two lengths of pitched cable are longitudinally displaced toward opposite sides and thereby displace the sunroof connected in suitable mechanical manner to the free ends of the two cables.
The two cables are guided within a tube, but it is easily seen that only a short length of the cable is used as a quasi gear-rack, with the remaining length of cable merely transmitting the force to the sunroof. This length also is laid in an arc around the corner.
It has long been known to be a drawback that the cable lengths which do not mesh with the pinion and, therefore, are not held or supported by it tend to knock in their guide tubes and, therefore, produce bothersome noises. It has long been desirable to tighten the cables within the guide tubes in order to eliminate this knocking. To date this problem has not been satisfactorily solved. For instance, an attempt has already been undertaken to cover the cable length not needed as a gear rack with plastic flocks of fibers. However this was found to be entirely unsuitable due to the softness of the fibers which are relatively quickly abraded during cable displacement at those sites of highest stress.
Again an attempt has been made to pull a shrinkable tube over the pitched cable, that is a plastic tube which shrinks upon subsequent heating and thereby tightly fits on the cable. However, the material suitable for shrinking also is found to be too soft and wears very rapidly in use.